1. Field of the Invention
The present invention relates to a ferroelectric capacitor and a semiconductor device having a ferroelectric capacitor.
2. Description of the Related Art
A FeRAM(ferroelectric random access memory) uses a ferroelectric capacitor. Data can be read quickly from the FeRAM, which can be operated to provide random access. Therefore, the FeRAM is expected as a new type of nonvolatile memory.
A memory cell of the FeRAM includes a switching transistor and a ferroelectric capacitor. The FeRAM uses a function of the ferroelectric layer for reversing an electric field by an intrinsic polarization and for retaining the electric field.
The FeRAM is classified broadly into either a planar type or a stack type. In the planar type FeRAM, a top electrode of the ferroelectric capacitor is connected to a source electrode of a corresponding switching transistor.
In the stack type FeRAM, a bottom electrode of the ferroelectric capacitor is connected to the source electrode of the switching transistor via a conductive plug. Therefore, an area of a memory cell of the stack type FeRAM is smaller than an area of a memory cell of the planar type FeRAM. Such technique is shown in a “A FRAM technology using 1T1C and triple metal layers for high performance and high density FRAMs”, S. Y. Lee et al., 1999 Symposium on VLSI Technology Digest of Technical Papers, 1999, pp. 141–142. Alternatively, a FeRAM structure that has a cross-sectional area of a bottom electrode smaller than a cross-sectional area of a ferroelectric layer is described in Japanese Patent Laid-Open No 2001-308287.
In the conventional FeRAM, a bottom electrode layer, a ferroelectric layer and a top electrode layer are formed in order, and then, these layers are etched all at once. However, a damaged layer might be formed on a side surface of the ferroelectric layer. The damaged layer is made by a reaction between a material of the ferroelectric layer, the top electrode or the bottom electrode and etching gas. If a damaged layer is formed, normal operation of the ferroelectric capacitor might be inhibited and reliability of the ferroelectric capacitor might not be ensured.
For solving the above problem, an alternative fabricating process for fabricating the FeRAM device is as follows. First, the bottom electrode layer is formed, and then the bottom electrode layer is etched to form the bottom electrode. Then, the ferroelectric layer is formed on the bottom electrode and the top electrode layer is formed on the ferroelectric layer. Then, the ferroelectric layer and the top electrode layer are etched to form the ferroelectric capacitor.
However, in the FeRAM device which is fabricated by these above steps, oxygen is diffused in an insulating layer formed under the bottom electrode, while the ferroelectric layer is formed under an oxygen atmosphere. As a result, a plug which is embedded in the insulating layer is oxidized and a connection between the bottom electrode and a source electrode of a switching transistor might be disconnected.
Also, a process that reestablishes the function of the ferroelectric layer by cleaning the damaged layer has been considered. However, an amount of remaining polarization is not increased after the cleaning. Such process therefore is not an effective solution.